HVDC power transmission has become increasingly important due to increasing need for power supply or delivery and interconnected power transmission and distribution systems. Power systems such as electrical power distribution or transmission systems generally include a protection system for protecting, monitoring and controlling the operation and/or functionality of other components included in the power system, which other components hence may be referred to as protected units. Such protection systems may for example be able to detect short circuits, overcurrents and overvoltages in power lines, transformers and/or other parts or components of the power system. The protection systems can include protection equipment such as circuit breakers for isolating any possible faults for example occurring in power transmission and distribution lines by opening or tripping the circuit breakers. After the fault has been cleared, e.g. by performing repairs and/or maintenance on the component in which the fault has been detected, the power flow can be restored by closing the circuit breakers.
Interface arrangements are known to be connected between an AC power system and a DC power system. Such an arrangement typically includes a converter, such as a voltage source converter, for conversion of AC power to DC power, or vice versa. The interface arrangement has a DC side for coupling to the DC power system and an AC side for coupling to the AC power system. The arrangement often includes a transformer having a primary side connected to the AC system and a secondary side for coupling to the converter.
For example in a HVDC power system, there is generally included an interface arrangement including or constituting a HVDC converter station, which is a type of station configured to convert high voltage DC to AC, or vice versa. An HVDC converter station may comprise a plurality of elements such as the converter itself (or a plurality of converters connected in series or in parallel), one or more transformers, capacitors, filters, and/or other auxiliary elements. Converters may comprise a plurality of solid-state based devices such as semiconductor devices and may be categorized as line-commutated converters, using e.g. thyristors as switches, or voltage source converters, using transistors such as insulated gate bipolar transistors (IGBTs) as switches (or switching devices). A plurality of solid-state semiconductor devices such as thyristors or IGBTs may be connected together, for instance in series, to form a building block, or cell, of an HVDC converter. The converter cell may in alternative be referred to as a (HVDC) converter valve.
In the interface arrangement there may also be provided an AC circuit breaker, which e.g. may be arranged between the primary windings of the transformer and the AC power system, for protecting the converter in case of occurrence of a fault by isolating the converter from the AC power system. However, in some cases, the fault current may contain a relatively large DC component, which may entail a relatively long delay before the fault current through the AC circuit breaker crosses zero, during which delay the AC circuit breaker cannot interrupt the fault current. Such a situation is in general undesirable and may be referred to as delayed zero-crossing. Delayed zero-crossing may make opening of the AC circuit breaker difficult or even impossible, and may lead to degradation or even malfunction of the AC circuit breaker.